1. Field of the Invention
The present invention relates, in general, to grids used for placing and supporting fuel rods in a nuclear reactor fuel assembly and, more particularly, to a grid with nozzle-type coolant deflecting channels for use in such a nuclear fuel assembly, the grid being so designed as to effectively deflect coolant, thus mixing lower temperature coolant with high temperature coolant within the assembly and improving heat transferring effect between the fuel rods and the coolant, the grid also protecting the fuel rods from fretting wear due to a swirling motion or a lateral circulation of the coolant, and being reduced in the thickness of the intersecting grid strips while maintaining a desired buckling strength of said strips, thus allowing the coolant to more smoothly flow in the assembly and thereby reducing pressure drop of the coolant while effectively resisting laterally directed forces acting on the grids.
2. Description of the Prior Art
In typical light water reactors, a plurality of elongated nuclear fuel rods 125 are regularly and parallelly arranged in an assembly 101 having a square cross-section in a way such that, for example, fourteen, fifteen, sixteen or seventeen fuel rods 125 are regularly arranged along each side of said square cross-section, thus forming a 14.times.14, 15.times.15, 16.times.16, or 17.times.17 array as shown in FIG. 1. In such a nuclear fuel assembly 101, the elongated fuel rods 125 are typically fabricated by containing a fissionable fuel material, such as uranium core, within a hermetically sealed elongated zircaloy tube 114, known as the cladding. In order to place and support such fuel rods 125 within the assembly 101, a plurality of spacer grids 110 are used. Each of such grids 110 is produced by welding a plurality of intersecting inner strips to each other into an egg-crate pattern prior to encircling the periphery of the grid 110 by four perimeter strips. The top and bottom of the fuel assembly 101 are, thereafter, covered with pallets 111 and 112, respectively. The assembly 101 is thus protected from any external loads acting on the top and bottom thereof. The spacer grids 110 and the pallets 111 and 112 are integrated into a single structure using a plurality of guide tubes 113. A framework of the assembly 101 is thus fabricated.
Each of the above spacer grids 110 is fabricated as follows. As best seen in FIG. 2, two sets of inner strips 115 and 116, individually having a plurality of notches at regularly spaced portions, are assembled with each other by intersecting the two sets of strips 115 and 116 at said notches, thus forming a plurality of four-walled cells individually having four intersections 117. The assembled strips 115 and 116 are, thereafter, welded together at said intersections 117 prior to being encircled by perimeter strips 118, thus forming a spacer grid 110 with such four-walled cells. As shown in FIG. 3, a plurality of positioning springs 119 and a plurality of positioning dimples 120 are integrally formed on or attached to the inner strips 115 and 116 in a way such that the springs 119 and the dimples 120 extend inwardly with respect to each of said four-walled cells. In such a case, the dimples 120 are more rigid than the springs 119. In each four-walled cell, the positioning springs 119 force a fuel rod 125 against associated dimples 120, thus elastically positioning and supporting the fuel rod 125 at four points within each of said cells. In the typical nuclear fuel assembly 101, a plurality of grids 110 are regularly and perpendicularly arranged along the axes of the fuel rods 125 at right angles, thus placing and supporting the fuel rods 125 at multiple points. That is, the grids 110 form a multi-point support means for placing and supporting the fuel rods 125 within a nuclear fuel assembly 101.
In such an assembly 101, the positioning springs 119 elastically and slightly force the fuel rods 119 against the dimples 120 in a way such that the fuel rods 125 are slidable on the support points of both the springs 119 and the dimples 120 when the fuel rods 125 are elongated due to irradiation induced growth during a circulation of the coolant within the assembly 101. When the fuel rods 125 are fixed to the grids 110 at the support points, the fuel rods 125 may be bent at portions between the support points of the grids 110, thus undesirably reducing the intervals between the fuel rods 125 of the assembly 101 as shown in FIG. 4.
In some typical nuclear reactors using water as coolant, water receives thermal energy from the fuel rods 125 prior to converting the thermal energy into electric energy. During an operation of a nuclear fuel assembly 101 of such a reactor, water or liquid coolant is primarily introduced into the assembly 101 through an opening formed on the core supporting lower plate of the reactor. In the assembly 101, the coolant flows upwardly through the passages, defined between the fuel rods 125, and receives thermal energy from the fuel rods 125. In such a case, the sectioned configuration of the coolant passages provided in the fuel assembly 101 is shown in FIG. 4.
Typically, the amounts of thermal energy generated from different nuclear fuel assemblies 101 are not equal to each other. Since the assemblies 101 individually have a rectangular configuration with the elongated, parallel fuel rods 125 being closely spaced apart from each other at irregular intervals, the temperature of coolant flowing around the fuel rods 125 is variable in accordance with positions. That is, the amount of thermal energy, received by water flowing around the corners 123 of each four-walled cell, is less than that received by water flowing around the fuel rods 125. The coolant passages of typical fuel assemblies 101 thus undesirably have low temperature regions. Such low temperature regions reduce the thermal efficiency of the nuclear reactor. The coolant passages of the fuel assemblies 101 may also have partially overheated regions at positions adjacent to the fuel rods 125 having a high temperature. Such partially overheated regions deteriorate soundness of the assemblies 101. In order to remove such partially overheated regions from a nuclear fuel assembly 101, it is necessary to design the grid in a way such that a uniform temperature distribution is formed in the fuel assembly 101. The grid is also designed to effectively deflect and mix the coolant within the assemblies 101. Such effectively mixed coolant makes uniform the increase in enthalpy and maximizes the core output. Typical examples of such designed grids are disclosed in Korean Patent Publication Nos. 91-1978 and 91-7921.
In the grids disclosed in the above Korean patents, so-called "mixing blades" or "vanes" are attached to the upper portion of each grid and are used for mixing coolants within the fuel assembly. That is, the mixing blades or vanes allow the coolant to flow laterally in addition to normally longitudinally, and so the coolants are effectively mixed with each other between the channels and between the lower temperature regions and the partially overheated regions of the fuel assembly.
On the other hand, a coolant mixing grid, comprising two sets of intersecting inner strips individually made up of two flat, narrow sheets deformed to provide channels for coolant, is disclosed in U.S. Pat. No. 4,726,926. In the above grid, the upper or lower portion of each channel is inclined relative to the axes of the fuel rods at an angle of inclination, thus producing a swirling motion of coolant at the inlet and outlet of said channels. Such a swirling motion of coolant improves the heat transferring effect between the fuel rods and the coolant within a nuclear fuel assembly.
The above Korean or U.S. grids, designed to form a lateral flow of coolant or to deflect and mix the coolant within a nuclear fuel assembly, are somewhat advantageous in that they more effectively mix the coolant and improve the heat transferring effect between the fuel rods and the coolant within a nuclear fuel assembly. However, such a grid is problematic in that the lateral flow or mixing of coolant regrettably vibrates the elongated, parallel, closely spaced fuel rods within the assembly. As described above, the fuel rods 125 are supported by both the positioning springs 119 and the positioning dimples 120 within the four-walled cells of the grids 110. However, during an operation of a nuclear fuel assembly 101, the fuel rods 125 quickly and periodically interfere with the intersecting strips of the grids due to vibrations caused by the lateral flow of coolant. When the fuel rods 125 are so vibrated for a lengthy period of time, the claddings of the fuel rods 125 are repeatedly and frictionally abraded at their contact parts at which the fuel rods 125 are brought into contact with the springs and dimples of the grids. The claddings are thus reduced in their thicknesses so as to be finally perforated at said contact parts. Such an abrasion of the fuel rods is so-called fretting wear in the art.
Such a fretting wear may be referred to Korean Patent Publication No. 94-3799 in detail. The laterally directed force caused by the mixing blades of the grids is in proportion to the coolant mixing effect and directly affects the thermal transferring effect of nuclear reactors. However, such a laterally directed force of the mixing blades also proportionally increases the amplitude of vibration of the fuel rods. This may cause damage to the fuel rods.
The important factors necessary to consider while designing the grids for use in nuclear fuel assemblies are improvement in both the fuel rod supporting function of the grids and the buckling strength resisting of such a laterally directed force acting on said grids. During an operation of a nuclear reactor, the fuel assemblies may be vibrated laterally due to a load acting on the assemblies and this causes an interference between the assemblies. Therefore, the grids of the fuel assemblies may be impacted due to such an interference between the assemblies as disclosed in U.S. Pat. No. 4,058,436. In the prior art, the grid's buckling strength, resisting a lateral load acting on the grid, is reduced since the grid strips have to be partially cut away through, for example, a stamping process at a plurality of portions so as to form positioning springs and dimples within a fuel assembly. Such cut-away portions reduce the effective cross-sectional area of the grid capable of resisting impact, thus reducing the buckling strength of the grid.
In a grid disclosed in U.S. Pat. No. 5,243,634, the positioning springs are individually integrated with an associated grid strip at one point, thus forming a cantilever structure. Such a cantilever spring is more flexible than a simple spring which is integrated with a grid strip at opposite ends thereof. In the mixing grid disclosed in the above-mentioned U.S. Pat. No. 4,726,926, the deformed portions, provided on the sheets of the intersecting grid strips for forming the channels for coolant, act as channel-shaped positioning springs used for placing and supporting the fuel rods within the four-walled cells. Since the sheets of the strips are not cut away but deformed to form such channel-shaped springs, flexibility of such channel-shaped springs is exceedingly less in comparison with the above-mentioned cantilever springs, thus failing to provide desired flexibility expected by conventional positioning springs. The channel-shaped springs thus act as dimples rather than springs. Therefore, the mixing grid, having such channel-shaped dimples, is problematic in that said dimples may cause the fuel rods to be undesirably bent when the fuel rods are elongated due to the irradiation induced growth during an operation of the reactor or to be scratched at the claddings when the fuel rods are inserted into the cells of the grids.